"He writes a book."
Translation:Él escribe un libro.
'Ello' means 'it' when the subject is unclear, and to say it's use is incorrect here is an understatement. 'Ello' is seldom used. It represents vague ideas or the neuter object of a proposition, like 'it' in “He swam as if his life depended on it." Ello is gender neutral, and therefore never means 'he'. Always use the proper gender-specific pronoun when the subject is known.